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Segmentation — Best Fit

In one sentence: Fit the optimal geometric primitive (plane, cylinder, or sphere) to each classified mesh segment and report the fit quality.

Overview

Workbench: Mesh
Menu: Mesh → Meshes → Segmentation — Best Fit…
Command: Mesh_SegmentationBestFit
Shortcut: none

An enhanced version of Segmentation (Curvature) that not only classifies mesh regions by surface type but also fits the optimal geometric primitive to each region and reports the RMS fit error.

Intuition

Where Segmentation (Curvature) just colours regions by type, Best Fit takes the next step: for each cylindrical region it computes the best-fit axis and radius; for each planar region it computes the best-fit normal and offset; for each spherical region it computes the best-fit centre and radius. These parameters can then be used directly to create exact B-rep primitives in the Part workbench for reverse engineering.

When to use it

  • Reverse engineering: you want to reconstruct an STL as exact B-rep geometry.
  • Quality inspection: you want to know how closely a scan matches a nominal primitive (cylinder, plane) and by how much it deviates.

When NOT to use it

  • Don't use it without running Segmentation first — Best Fit requires pre-classified segments from Segmentation (Curvature).

Step-by-step

  1. Run Segmentation (Curvature) to classify the mesh.
  2. Choose Mesh → Meshes → Segmentation — Best Fit….
  3. Review the fitted primitives and their RMS fit errors.
  4. Use the reported parameters (radius, axis, normal) to create exact Part workbench geometry.

Supported primitives

Primitive Parameters computed
Plane Normal vector, D (distance from origin), RMS fit error
Cylinder Axis direction, Axis point, Radius, RMS fit error
Sphere Centre point, Radius, RMS fit error

Fit quality interpretation

RMS fit error Interpretation
< 0.1 mm Excellent — the segment closely matches the primitive
0.1–0.5 mm Acceptable for rough reconstruction
> 0.5 mm Poor — the segment may not be the assumed type, or mesh resolution is too coarse

Common mistakes and pitfalls

Best Fit requires pre-classified segments

Run Segmentation (Curvature) first to identify region types. Best Fit operates on those classified segments.

High RMS error may mean wrong primitive type

If a region has RMS > 0.5 mm for a cylindrical fit, the region may be a cone, torus, or freeform surface rather than a cylinder. Review the segmentation classification.

See also